An NYPD detective (Jim Caviezel) uses an old ham radio to communicate back in time and save his firefighter dad (Dennis Quaid) from being killed in 1969. Quaid’s superlative acting and Gregory Hoblit’s taut direction make for an exciting and affecting sci-fi thriller – till things get convoluted in the second half.Running time: 121 minutes. Rated: PG-13. At the Empire, the Lincoln Square, the Union Square, others.

ONE of the reasons I love movies is that the right actor and director can make you believe passionately in the wildest storyline – and “Frequency” had me watching through misty eyes, at least for the first half.

For your consideration, as Rod Serling used to say on “The Twilight Zone,” I give you NYPD Detective John Sullivan (Jim Caviezel of “The Thin Red Line”). He has a perpetually hurt look, drinks too much, and his girlfriend has walked out on him.

Before you can say “It’s a Wonderful Life,” he discovers a great gift in a closet of the Bayside, Queens, home where he’s lived his entire life. It’s an old ham radio on which, through some celestial quirk, he’s able to contact his late father, Frank (Dennis Quaid).

Better still, John starts chatting with the old man, a firefighter, on the day before the defining event of the son’s childhood – dad’s death in an awful blaze 30 years earlier.

John is able to wise up an unbelieving Frank and save his father’s life. Suddenly, pictures of a middle-aged Frank appear on the walls – and there’s not likely to be a dry eye in the audience.

But messing with the past has its consequences, as any science-fiction fan knows. It’s giving away a lot less than the spoiler-filled trailer to report that Frank’s survival helps a serial killer – whose victims will include John’s mom (Elizabeth Mitchell), unless father and son, working together, can alter the past again.

It was there that the movie started to let me down. I was buying it, largely thanks to Quaid’s stellar work and the skill of the director (Gregory Hoblit of “Primal Fear”) at shifting gears from nail-biting suspense to male-weepy scenes that put “Field of Dreams” to shame.

But the plot becomes ever-more complicated and credibility-defying. I started wondering: How on earth can they ever end this in a dramatically satisfying fashion?

The you-can-have-it-all ending that they settled on didn’t work for me, but I’ll be darned if I can think of a better one.

The movie’s flawless evocation of 1969 Queens, where I grew up – the Mets’ World Series’ victory figures prominently in Toby Emmerich’s script, which has many subtle touches and a neat joke about Yahoo – deeply resonated with me, as did the movie’s theme.

Quaid, one of our most underrated actors, has rarely been better – and he has great chemistry with Caviezel.

My head says “Frequency” has a 31/2-star first half and 21/2-star second half, averaging out to three stars.